package cn.icanci.rec.admin.dal.utils.service;

/**
 * id生成器-雪花算法
 *
 * snowflake的结构如下(每部分用-分开):
 * 0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000
 * 第一位为未使用，接下来的41位为毫秒级时间(41位的长度可以使用69年)，然后是5位datacenterId和5位workerId(10位的长度最多支持部署1024个节点） ，最后12位是毫秒内的计数（12位的计数顺序号支持每个节点每毫秒产生4096个ID序号）
 * 一共加起来刚好64位，为一个Long型。(转换成字符串长度为18)
 * snowflake生成的ID整体上按照时间自增排序，并且整个分布式系统内不会产生ID碰撞（由datacenter和workerId作区分），并且效率较高。据说：snowflake每秒能够产生26万个ID。
 *
 * @author icanci
 * @since 1.0 Created in 2022/10/30 07:04
 */
public class SnowFlake {
    /** 起始的时间戳 */
    private static final long START_STAMP        = 1480166465631L;

    /** 序列号占用的位数 */
    private static final long SEQUENCE_BIT       = 12;

    /** 机器标识占用的位数 */
    private static final long MACHINE_BIT        = 5;

    /** 数据中心占用的位数 */
    private static final long DATACENTER_BIT     = 5;

    /** 每一部分的最大值 */
    private static final long MAX_DATACENTER_NUM = ~(-1L << DATACENTER_BIT);

    private static final long MAX_MACHINE_NUM    = ~(-1L << MACHINE_BIT);

    private static final long MAX_SEQUENCE       = ~(-1L << SEQUENCE_BIT);

    /** 每一部分向左的位移 */
    private static final long MACHINE_LEFT       = SEQUENCE_BIT;

    private static final long DATACENTER_LEFT    = SEQUENCE_BIT + MACHINE_BIT;

    private static final long TIMESTAMP_LEFT     = DATACENTER_LEFT + DATACENTER_BIT;

    /** 数据中心 */
    private final long        datacenterId;

    /** 机器标识 */
    private final long        machineId;

    /** 序列号 */
    private long              sequence           = 0L;

    /** 上一次时间戳 */
    private long              lastStamp          = -1L;

    public SnowFlake(long datacenterId, long machineId) {
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            throw new IllegalArgumentException("datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
        }
        this.datacenterId = datacenterId;
        this.machineId = machineId;
    }

    /**
     * 产生下一个ID
     *
     * @return 下一个ID
     */
    public synchronized long nextId() {
        long currStamp = getNewStamp();
        if (currStamp < lastStamp) {
            throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
        }
        if (currStamp == lastStamp) {
            //相同毫秒内，序列号自增
            sequence = (sequence + 1) & MAX_SEQUENCE;
            //同一毫秒的序列数已经达到最大
            if (sequence == 0L) {
                currStamp = getNextMill();
            }
        } else {
            //不同毫秒内，序列号置为0
            sequence = 0L;
        }
        lastStamp = currStamp;
        return //时间戳部分
        (currStamp - START_STAMP) << TIMESTAMP_LEFT
               //数据中心部分         
               | datacenterId << DATACENTER_LEFT
               //机器标识部分         
               | machineId << MACHINE_LEFT
               //序列号部分
               | sequence;
    }

    private long getNextMill() {
        long mill = getNewStamp();
        while (mill <= lastStamp) {
            mill = getNewStamp();
        }
        return mill;
    }

    private long getNewStamp() {
        return System.currentTimeMillis();
    }
}